Founded in 1934 and responsible for films in many genres, Hammer Film Productions is best known for its horror output, starting with the adapted-from-television SF of The Quatermass Xperiment and then overarched by a run of titles, rooted in Victorian Gothic, that often star Peter Cushing and Christopher Lee. The production house’s reign of terror was seriously challenged when films like Roman Polanski’s Rosemary’s Baby and George A. Romero’s Night Of The Living Dead were made in the United States – for these films, shifting their spooky scenarios from the old European castle to the modern American home and appealing to an emerging youth counterculture, were ushering in a new golden age of horror that threatened Hammer’s very relevance, let alone dominance, in the marketplace. Still, for a relatively brief period the decline of Hammer would be put in abeyance by a distribution alliance with Warner Bros (now owned by Hammer’s long-term American partners Seven Arts) that guaranteed greater investments and wider audiences.

Marcus Hearn’s documentary Hammer Horror: The Warner Bros Yearscovers this rapidly evolving climate, and is as much elegy for as celebration of the Hammer style and spirit. The irony is that even as Hammer tried to get with the times and inject new blood into its creaky old brand, introducing plots involving illegal drug use (Terence Fisher’s Frankenstein Must Be Destroyed; Alan Gibson’s thriller Crescendo), incorporating groovy scores (Michael Carreras’ Moon Zero Two), depicting older generations monstrously preying on the young (Peter Sasdy’s Taste the Blood Of Dracula), and eventually bringing Dracula himself into (an old-fashioned and hilariously inauthentic version of) the present day (Alan Gibson’s Dracula AD 1972 and The Satanic Rites Of Dracula) while amping up the films’ sexual content, Warner was marketing their films, starting with Terence Fisher’s Dracula Has Risen From The Grave, to America’s hipster audiences as cheesily fusty affairs whose very outmodedness was part of their camp appeal. Indeed, as Hammer tried to recruit new talent (including actors Veronica Carlson, Madeline Smith and Caroline Munro, and director Sasdy, all charming interviewees here), Warner seemed only to want more vampire vehicles for the ageing Lee and Cushing, practically forcing Hammer to become a producer of heritage cinema. Attempts to branch out and to mix up genres – as with the sc-fi western Moon Zero Two or Roy Ward Baker’s horror/martial arts crossover The Legend Of The 7 Golden Vampires – failed to find their audience.

Something had to give, and it would all come to an unhappy end when Warner found immense box-office success with its home-grown, entirely un-Hammer-like The Exorcist (1974), and would lose interest in Hammer’s rather tired looking output. It did not help that by this point Seven Arts had long since ceased to own Warner. Left without its big American backers, Hammer would suffer from its reduced budgets and moribund ideas – although Hearn’s documentary ends with a teaser of what might have been, showing the resurrection of unfilmed, India-set Hammer script The Unquenchable Thirst Of Dracula at the Mayhem Film Festival, where the screenplay was read live.

Formally a rather conventional documentary, Hammer Horror: The Warner Bros Years switches between film clips and talking heads, on-set anecdotes and scholarly contextualisation – but it is a winning mix, finding many points of interest even for non-fans of the films themselves. Indeed, part of what is so appealing about the critical assessments here – coming from Christopher Frayling (Nightmare: The Birth of Horror), Denis Meikle (A History of Horrors), IQ Hunter (British Trash Cinema), Wayne Kinsey & Gordon Thompson (Hammer Films On Location), as well as from filmmaker/fan Joe Dante (Piranha, The Howling, Gremlins, The ‘Burbs, Matinee) – is their honesty. All these contributors are, broadly, enthusiasts as much as experts, but they are not offering hagiography, and are frank on the many shortcomings of the films under discussion.

Granted particular prominence, the wryly understated, dulcet-toned commentator Jonathan Rigby (author of English Gothic) speaks to camera in a natural history museum, with the skeleton of a large animal behind him rather pointedly aiming its large teeth at his neck. It is an apt visual metaphor both for Hammer’s vampiric fixations, and for its eventual fossilisation. For it is almost as though another of the titles from the Warner years, Val Guest’s When Dinosaurs Ruled The Earth might also describe the peak period of Hammer horror, before the extinction set in.